The present invention relates to an IrDA (infrared data association) modulation/demodulation integrated circuit device as is used in a portable telephone to achieve infrared data communication.
A conventional IrDA modulation/demodulation integrated circuit device will be described with reference to FIG. 5. A portable telephone 50 incorporates a telephone CPU (central processing unit) 52 for controlling the entire telephone, a base-band integrated circuit device 55 for processing a base-band signal, and other components.
In addition, to achieve IrDA-complying infrared data communication with an external personal computer 60 or the like, the portable telephone 50 also incorporates an IrDA modulation/demodulation integrated circuit device 58. For example, the IrDA modulation/demodulation integrated circuit device 58 is used to transfer facsimile data stored in the personal computer 60 to the portable telephone 50 by infrared rays so as to allow the thus transferred data to be further transferred from the portable telephone 50 to a remote location by radio waves. Of course, it is also possible to perform communication simply between the portable telephone 50 and the personal computer 60.
The IrDA modulation/demodulation integrated circuit device 58 modulates a signal that is going to be transmitted by infrared rays, and feeds the modulated signal to an analog front end 4. The analog front end 4, by radiating infrared rays from a light-emitting diode or the like, transfers the signal to the personal computer 60 or the like as indicated by the arrow A. When, in return, the signal transferred from the personal computer 60 by infrared rays as indicated by the arrow B is received by a photodiode or the like provided in the analog front end 4, the analog front end 4 first performs waveform shaping on the received signal, and then feeds the signal to the IrDA modulation/demodulation integrated circuit device 58. The IrDA modulation/demodulation integrated circuit device 58 then demodulates the signal.
However, the CPU 52, the integrated circuit device 55, and the integrated circuit device 58 use clocks of different frequencies, and thus require provision of separate crystal resonators (oscillators)51, 54, and 57. Consequently, the CPU 52, the integrated circuit device 55, and the integrated circuit device 58 require provision of separate oscillation circuits 53, 56, and 59. Although not shown, the portable telephone 50 may even incorporate a further integrated circuit device that requires provision of a separate resonator of its own.
The clock originating from the resonator 54 has a frequency of, for example, 12.6 MHz, 12.8 MHz, or 14.4 MHz. This clock is used by the CPU (not shown) provided within the base-band IC 55 and others. On the other hand, the clock used in the IrDA modulation/demodulation integrated circuit device 58 has a frequency of, for example, 153.6 kHz, 3.6864 MHz, or 7.3728 MHz. These values are based on the fact that IrDA standard baud rate is 9.6 kbps, and are thus set equal to a whole number times 9.6 kHz to allow a clock having a frequency of 9.6 kHz to be produced easily by frequency division.
In this way, the CPU 52, the IC 55, and the integrated circuit device 58 use separate clocks, and thus require provision of separate resonators 51, 54, and 57. That is, incorporating the IrDA modulation/demodulation integrated circuit device 58 into the portable telephone 50 leads to an increase in the number of resonators that need to be incorporated therein. Thus, the conventional portable telephone 50 described above demands comparatively high cost, and requires a circuit board having a comparatively large area to allow provision of those resonators.
An object of the present invention is to provide an IrDA modulation/demodulation integrated circuit device that, by reducing the number of resonators incorporated in a portable telephone, helps reduce the cost thereof and reduce the area of the circuit board provided therein.
To achieve the above object, according to a first configuration of the present invention, an IrDA modulation/demodulation integrated circuit device designed to be incorporated in a portable telephone is provided with a PLL circuit that receives a clock used in a base-band integrated circuit device for processing a base-band signal and that converts the frequency of the received clock to produce a clock for the IrDA modulation/demodulation integrated circuit device.
According to this configuration, the IrDA modulation/demodulation integrated circuit device takes in the clock (having a frequency of, for example 12.6 MHz) used in the base-band integrated circuit device, and converts its frequency to, for example, 7.3728 MHz by using the PLL circuit. The IrDA modulation/demodulation integrated circuit device modulates or demodulates a signal by using the clock that has undergone such conversion.
According to a second configuration of the present invention, in the IrDA modulation/demodulation integrated circuit device of the first configuration described above, the PLL circuit is provided with a first frequency division circuit for dividing the frequency of the clock used in the base-band integrated circuit device by a factor of n (where n is a whole number), a phase comparator for detecting the phase difference between the output of the first frequency division circuit and the output of a second frequency division circuit, a low-pass filter for eliminating a high-frequency component from the output of the phase comparator, and a voltage-controlled oscillator whose oscillation frequency is controlled by the output of the low-pass filter. In addition, a control means is provided that divides the output of the voltage-controlled oscillator by a factor of m (where m is a whole number) by using the second frequency division circuit and that can select a specific value of n from among one or more values and a specific value of m from among one or more values.
According to this configuration, even if the frequency of the clock varies from one base-band integrated circuit device to another, it is possible to keep constant the frequency of the clock produced by the PLL circuit by selecting appropriate values of n and m by using the control means. The control means is realized, for example by controlling selectors by using control registers. In accordance with the base-band clock frequency, a selector switches signal paths in such a way as to vary the value of n in the first frequency division circuit. A similar selector is provided also on the input side of the second frequency division circuit so as to vary the value of m.
According to a third configuration of the invention, in an IrDA modulation/demodulation integrated circuit device of the second configuration described above, the control means selects values of n and m that satisfy a relation m/n=8xc3x97k/875 (where k is a whole number) when the clock of the base-band signal has a frequency of 12.6 MHz, selects values of n and m that satisfy a relation m/n=9xc3x97k/1000 when the clock of the base-band signal has a frequency of 12.8 MHz, and selects values of n and m that satisfy a relation m/n=k/125 when the clock of the base-band signal has a frequency of 14.4 MHz
According to this configuration, whichever frequency, namely 12.6 MHz, 12.8 MHz, or 14.4 MHz, the clock of the base-band integrated circuit device may have, by selecting values of m and n that satisfy one of the above-noted relations, it is possible to obtain from the PLL circuit a clock having a frequency, for example 7.3728 MHz, that is equal to 115.2 kHz multiplied by k.
According to a fourth configuration of the present invention, in an IrDA modulation/demodulation integrated circuit device of the second or third configuration described above, the IrDA modulation/demodulation integrated circuit device is connected to an analog front end that emits and senses infrared rays, and the IrDA modulation/demodulation integrated circuit device starts operating by being started by a starting circuit provided therein when the analog front end senses the infrared rays.
According to this configuration, when a personal computer or the like attempts to communicate with the portable telephone by infrared rays, it first performs a discovery communication process in which it outputs a signal searching for a communication partner at regular intervals. When the analog front end senses this signal, it feeds the signal to the IrDA modulation/demodulation integrated circuit device. Designed to be able to refer to this signal, the starting circuit then starts the operation of the entire IrDA modulation/demodulation integrated circuit device. In this way, the portable telephone starts communicating with the personal computer or the like.
According to a fifth configuration of the invention, an IrDA modulation/demodulation integrated circuit device designed to be incorporated in a portable telephone is provided with a PLL circuit that receives a clock used in a base-band integrated circuit device for processing a base-band signal and that converts the frequency of the received clock, a frequency division circuit for dividing the frequency of the clock output from the PLL circuit, a reception circuit for demodulating an IrDA-complying signal fed thereto from the outside by infrared rays by using the clock output from the frequency division circuit, and a control register for changing the frequency division factor of a frequency division circuit provided within the PLL circuit when the reception circuit fails to demodulate the IrDA-complying signal.
According to this configuration, the clock produced by the PLL circuit after the entire integrated circuit device has been started is subjected to frequency division by the frequency division circuit and then fed to the reception circuit. If the reception circuit fails to perform communication at that frequency, it notifies the control register of the failure of reception. The control register is a register for setting the frequency division factor of the frequency division circuit of the PLL circuit, and, if reception fails, it changes the frequency division factor of the PLL circuit and thereby changes the clock frequency. If reception succeeds, the setting made by the control register is maintained to keep constant the frequency of the clock output from the PLL circuit; if reception fails again, the control register further changes the clock frequency.